(1) Field of the Invention
The present invention relates to a technique of displaying a character image of a vector font on a display unit.
(2) Description of the Related Art
In general, a portable type information processing apparatus such as portable telephone or PDA (Personal Digital Assistant) employs dot fonts for the display of characters because of a low throughput or graphic throughput of a CPU (Central Processing Unit).
FIG. 19 is an illustration of an example of a dot font. The dot font is for expressing a character image as information on the presence or absence of dots, and the display of a character image is feasible without conducting complicated arithmetic operations only by developing the dot font, stored in a memory or the like in advance, on a screen and, hence, the dot font is an efficient means for use in the portable type information processing apparatus having a low throughput as mentioned above. However, as shown in FIG. 19, the character expression based on a binary image of a dot font cannot provide a character image with a high-quality design.
Meanwhile, the “Effects of Character Size, Character Format, Pixel Density on Subjective Legibility of Reflective Liquid Crystal Displays for Personal Digital Assistants” (written by Satoru Kubota; Image Information Media Academic Journal, Vol. 55, No. 10, pp. 1363-1366, 2001. 10) says that a character of approximately 3.5 mm is clearly visible on a liquid crystal screen.
For example, in general, the display resolution of a liquid crystal panel for use in a portable telephone is approximately 140 dpi (dot per inch), and in a case in which a character of approximately 3.5 mm is displayed on this liquid crystal panel with approximately 140 dpi, the pixels constituting one character are approximately 20 dots×20 dots. Moreover, the number of strokes (number of edges) of a Japanese character becomes approximately 10 to 12 in the case of a larger number of strokes. Accordingly, when a complicated character having many strokes is displayed on a liquid crystal panel of a portable telephone, there is a need to deform the character itself, such as omitting a portion of the character.
Recently, a liquid crystal panel having a display resolution exceeding 180 dpi has been employed for built-in equipment such as a portable type information processing apparatus, and a portable type information processing apparatus has appeared which has a throughout (level equal to PC approximately 10 years ago) capable of operating a graphic library. Moreover, also for portable telephones, there has been an increased need for a high-quality design.
On the other hand, in the case of a dot font, since a fine font having a large number of dots requires a large volume of data, the character image capacity increases along with the trend to higher definition of a display screen of a portable type information processing apparatus, which creates a problem of the shortage of storage capacity needed for font data.
Meanwhile, as a common method for the faithful reproduction of a character design, curves constituting a character are converted into data and the character form is reproduced in accordance with a character size to be supplied. The outline font is one example of this method. There is an increased need to mount this outline font in a built-in equipment. FIG. 20 is an illustration of an example of an outline font.
Referring to a flow chart (steps A10 to A50) of FIG. 21, a description will be given hereinbelow of a character image production process based on an outline font.
First of all, outline data such as font data, UNICODE and curve data are read out from a font memory or the like (step A10), and an outline is produced on the basis of this outline data (step A20). Moreover, anti-aliasing area is produced on the basis of the produced outline (step A30), then writing the outline (reproduction of a character outline) (step A40). Still moreover, a portion inside the outline is painted out (step A50) so as to create a character image.
The outline font outline is converted into data as straight lines and approximate curves. Moreover, as the typical approximate curves, there are the Bezier curve and the spline curve. For example, in the case of the character image generation using the Bezier curve, the starting point, the end point and control points for the interpolation therebetween are used as data and, after a closed curve is obtained on the basis of these points, the internal portion of the closed curve is painted out to generate a character image. That is, in the case of the outline font, a character outline is generated on the basis of some control point information.
FIG. 22 is an illustration of a functional expression on the Bezier curve. In general, the Bezier curve is a curve to be used for the PostScript or the like, and the Bezier function to be used for this Bezier curve is a function representing a tertiary approximate curve. When the starting point of a curve, the end point thereof and two control points thereof are taken as (x1, y1), (x4, y4) and (x2, y2), (x3, y3), respectively, as shown in FIG. 22, the Bezier curve is expressed by the following equations (1) and (2).x=(1−t)3x1+3(1−t)2tx2+3(1−t)t2x3+t3x4  (1)y=(1−t)3y1+3(1−t)2ty2+3(1−t)t2y3+t3y4  (2)where 0≦t≦1
FIG. 23 is an illustration of a functional expression on the spline curve. In general, the spline curve is an approximate curve to be used for the True Type font or the like, and the spline function to be used for this spline curve is a function representing a secondary approximate curve. When the starting point of a curve, the end point thereof and the control point thereof are taken as (x1, y1), (x3, y3) and (x2, y2), respectively, as shown in FIG. 23, the spline curve is expressed by the following equations (3) and (4).x=(1−t)2x1+2t(1−t)x2+tx3  (3)y=(1−t)2y1+2t(1−t)y2+ty3  (4)where 0≦t≦1
Incidentally, as a font for forming a character image according to a method similar to the outline font mentioned above, there is the stroke font expressing a glyph configuration by a vector of a skeleton line of image lines thereof. This stroke font is for producing a character image, for example, by retaining a center line of a character and thickening this center line.
[Non-Patent Document 1] written by Satoru Kubota “Effects of Character Size, Character Format, Pixel Density on Subjective Legibility of Reflective Liquid Crystal Displays for Personal Digital Assistants” (Image Information Media Academic Journal, Vol. 55, No. 10, pp. 1363-1366, 2001. 10)
In general, for carrying out the character display in an information processing apparatus, it is desirable that the processing from the generation of a character image to the display on a display unit is conducted within a time shorter than a time (approximately 0.2 seconds) which is detectable by the human being.
However, a conventional outline font generation method requires the calculation processing of a quadratic or cubic equation even in a case in which the generation of a border line (outline) is made through the use of any one of the Bezier curve or the spline curve, which requires much time for the outline generation processing. In particular, since a portable type information processing apparatus such as portable telephone has a low throughput, the outline generation processing takes a long time to display a character image based on the outline font.
Thus, in the outline font generation process, it is desirable to conduct the outline generation processing, which requires time particularly, within a short time.